1. Field of the Invention
The present invention relates to a location registration control method implemented in a mobile communications system. More particularly, this invention relates to a location registration control method utilizing variable location registration areas.
2. Description of the Related Art
Location registration control is needed in a mobile communications system. A mobile station makes location registration in a location registration area including a plurality of radio zones, one of which the mobile station is located in. When an incoming call addressed to the mobile station occurs, a selective calling signal is transmitted to the mobile station located within the radio zone belonging to the location registration area in which the mobile station has made the location registration.
To suppress an increase in the frequency of occurrence of a location registration request (a traffic for location registration), there has been proposed a location registration method utilizing variable location registration areas that are controlled so that the size thereof varies depending on a situation (see Japanese Unexamined Patent Publication No. 8-205228).
More specifically, during some of time zones in a day, many mobile stations are put in use. During such a time zone, a location registration request signal is transmitted frequently. However, the frequency of transmitting the location registration request signal must be made as small as possible even during the time zone. For this reason, it is necessary to widen the range of one location registration area. By contrast, during some time zones, mobile stations do not move very largely but dwell in the same radio zone. During such a less-movement time zone, it is required that the range of one location registration area must be narrowed so that paging can be achieved readily and reliably at the time of incoming call occurrence. Even when the range of one location registration area is narrow, if the mobile stations move a little, then it is unnecessary to care about an increase in the traffic for location registration.
The location registration method disclosed in the Japanese Unexamined Patent Publication No. 8-205228 is a technique based on the foregoing idea. However, when the arrangement of the location registration areas is changed, a larger number of mobile stations must have their location registrations corrected. This poses a problem in that the traffic for location registration increases abruptly.
To solve the above problem, a location registration control method has been proposed in efforts (see Japanese Unexamined Patent Publication No. 9-18938). According to this conventional control method, each base station transmits a location registration area conditional expression and location registration area information to the mobile stations located therein. The location registration area conditional expression gives a condition that must be satisfied by each radio zone belonging to a location registration area. The location registration area information is given to each radio base station. A mobile station stores information of a radio zone (registration zone) in which the mobile station has made a location registration, that is, identification information of the radio zone. Unless a relationship between the registration zone information and the location registration area information satisfies the location registration area conditional expression, the mobile station is judged to have left the location registration area in which the mobile station has made a location registration. Therefore, the location registration will be made again.
Accordingly, when location registration areas are restructured, mobile stations satisfying the condition given by the location registration area conditional expression need not modify their location registrations. Consequently, an abrupt increase in the traffic for location registration can be suppressed.
However, according to the technology illustrated in the Japanese Unexamined Patent Publication No. 9-18938, when location registration areas are restructured, some mobile stations cannot satisfy the condition given by the location registration area conditional expression. If many mobile stations cannot satisfy the condition given by the location registration area conditional expression, then the traffic for location registration increases when location registration areas are restructured.
An object of the present invention is to provide a location registration control method which can dramatically reduce the traffic for location registration.
Another object of the present invention is to provide a location registration control method which eliminates the need for a location registration request of any mobile station at the time when the location registration areas are restructured.
According to the present invention, in a mobile communications system composed of a control station, a plurality of radio base stations each forming a radio zone, and a plurality of mobile stations, the control station changes a combination of radio zones to form a configuration of location registration areas each formed by at least one radio zone. The control station stores a history of changes in the configuration of location registration areas each having a location code assigned thereto in a location registration area table. The control station then transmits a changed location code to a base station included in a location registration area associated with the changed location code.
The base station receiving the changed location code changes a location code of a broadcast channel to the changed location code, and broadcasts the changed location code on the broadcast channel.
A mobile station receives a broadcast channel when the mobile station has moved from a first radio zone of a first base station to a second radio zone of a second base station. When receiving the broadcast channel, the mobile station compares a first location code of the first base station with a second location code of the second base station. When the first location code matches the second location code, the mobile station receives an incoming call notification channel from the second base station without making a location registration request. When the first location code does not match the second location code, the mobile station makes a location registration request through the second base station.
The control station may store a zone identification code of a base station where the mobile station has made a location registration request and a time instant when the location registration request has been made onto a location registration acceptance table. When an incoming call addressed to a certain mobile station occurs, the control station searches the location registration area table and the location registration acceptance table for location codes of location registration areas including a radio zone in which the certain mobile station is currently located. And the control station instructs all base stations associated with the location codes searched for to transmit an incoming call notification signal.